This description relates to land/groove track type and pickup head movement direction detection.
FIG. 1 shows an example of an optical recording system 10 for recording data to and reading data from an optical disc 12. The recording system 10 includes a pickup head 20 that has a laser diode for generating a laser beam 30 and lens (not shown) for focusing the laser beam 30 onto the disc 12. A disc drive controller 14 controls a spindle motor 16 and a sled motor 18, in which the spindle motor 16 adjusts the rotational speed of the disc 12, and the sled motor 18 moves the pickup head 20 over larger distances along a radial direction across the disc 12. The pickup head 20 includes focusing and tracking actuators (not shown), in which the focusing actuator adjusts the position of the lens in an axial direction of the beam 30 to focus the beam 30 on the tracks of the disc 12, and the tracking actuator moves the lens over smaller distances (e.g., several tracks), allowing fine-tuning of the radial position of the laser beam. The position of the beam 30 relative to the disc 12 in the radial direction is controlled by a combination of the sled motor 18 and the tracking actuator. The controller 14 includes circuitry for encoding signals written to the disc 12, circuitry for decoding signals retrieved from the disc 12, and circuitry for interfacing with a host computer 19.
FIG. 2 shows an example of the optical disc 12 that includes a groove track 22 and a land track 28, in which each track forms a spiral on the disc 12. The spiral has multiple turns. In the description below, the plural noun “tracks” may refer to the groove and land tracks, multiple turns of a groove track, or multiple turns of a land track.
FIG. 3 shows a perspective view of the land tracks 28 and the groove tracks 22. The tracks guide the pickup head 20 during read and write operations. Data is written in the tracks by modifying the reflectances of portions of the tracks. As the pickup head 20 scans the tracks, the laser beam 30 is reflected from the tracks, and the intensity of the reflected laser beam is modulated according to the data written in the tracks. The borders of the tracks have recurring deviations in a radial direction 50, referred to as wobbles. The disc 12 may include one or more additional layers not shown in FIG. 3, such as a recordable layer or a rewriteable layer, a reflective layer, and a protective layer.
In one example, data is stored in the groove tracks, and the wobbles in the borders of a groove track 22 include a sinusoidal deviation that is modulated to contain address information. As the pickup head 20 scans the tracks, the reflected laser beam 30 is also modulated by the track wobble, from which a wobble signal that contains information about the track wobble can be generated. The wobble signal can be demodulated to retrieve the address information, which is used by the system 10 to position the pickup head 20 at particular locations in the groove track.
To write data to or read data at a specified address on the disc 12, the system 10 locks the laser beam 30 onto a specified groove track and searches for the specified address. Locking the laser beam 30 to a particular groove track is made difficult by disc run-out problems caused by misalignment and eccentricity of the disc 12.
Referring to FIG. 4, due to manufacturing tolerances, the tracks on the optical disc 12 may not be concentric to a center 56 of a center hole 58 of the disc 12. Also, due to tolerances in the placement of the disc 12 within the recording system 10, the center 56 of the disc 12 may not be perfectly aligned with an axis of rotation of the disc (which is aligned with a center axis of the spindle motor 16). As a result, when the disc 12 rotates, the beam 30 may not follow the groove track closely, but rather, move from an inner track (e.g., at position P1) to an outer track (e.g., at position P2), and from the outer track back to the inner track. The shaded spots represent different positions on the disc 12 on which the laser beam 30 is projected as the disc rotates one revolution.
Moving the laser beam 30 relative to the tracks involves the control of the sled motor 18 and the tracking actuator. For simplicity of description, only the description for the control of the pickup head is provided, and the description for the control of the tracking actuator is omitted. By saying that the pickup head 20 is at a particular track, we mean that the positions of the pickup head 20 and the lens are controlled so that the center of the laser beam 30 is at the particular track, in which a portion of the laser beam 30 may cover an adjacent track. By saying that the pickup head 20 is locked on a particular track, we mean that the positions of the pickup head 20 and the lens are controlled so that the laser beam 30 is locked on the particular track.
Knowing whether the pickup head 20 is currently at a groove track or a land track, and whether the pickup head 20 is moving from an inner track to an outer track, or from an outer track to an inner track, can assist the optical recording system 10 in locking the pickup head 20 on a particular track using a control feedback loop. A tracking error signal can be derived from output signals of photo detectors that detect the reflected laser beam 30. The tracking error signal can be used to determine whether the pickup head 20 is at the center of a track. In one example, the tracking error signal becomes zero when the pickup head 20 is at the center of a land track 28 or a groove track 22, and has a larger or smaller value when the pickup head 20 deviates from the center of the tracks. The optical recording system 10 cannot determine whether the pickup head 20 is at a land track 28 or a groove track 22 based on the tracking error signal alone.